INHERITANCE OF PARENTAL MODIFICATIONS 67 



gation. If it be true that closely related individuals, when 

 bred together, are more fertile than are distant relatives, 

 as seems under some circumstances to be true, this fact 

 also is of great importance. The whole subject of physio- 

 logical selection needs much more study. It is surely of 

 some importance as a cause of segregation ; it may be of 

 great importance. 



Segregation might cause the perpetuation of divergent 

 characters, though these were of no use and so not subject 

 to the preserving action of natural selection. This, how- 

 ever, would not produce adaptation to the environment, 

 which is the striking character of animals and plants. 

 Segregation, therefore, unaided by natural selection, cannot 

 have been an important factor in that evolution of animals 

 and plants which we find has taken place, bringing them 

 into harmony with their environment. Segregation becomes 

 important when it acts in connection with the other factors 

 of evolution, natural selection, sexual selection, and, possibly 

 also, among lowly forms, in connection with the inheritance 

 of parental modifications. 



THE INHERITANCE OF PARENTAL MODIFICATIONS 



One more factor in evolution needs careful discussion, 

 namely, the inheritance of parental modifications, that 

 which Weismann has called the "inheritance of acquired 

 characters." For this factor the largest claims are made 

 by some biologists. It probably exerts a powerful influence 

 on the evolution of some of the lower forms of plants and 

 animals. Its influence upon higher forms is much more 

 doubtful. 



